Caster hybrids that have a spec that does not or to a lower extent uses mana will get the passive ability to increase their manapool by 400% for specs that do use it.
Why those hybrids have such high base mana costs is because their base mana is still the 1/5 value of that increased manapool.
In order to compensate and make heals equally expensive they artificially scale the base mana cost to reflect the cost of pure mana users.
So 4% of 100k is still the same as 25% of 20k.
There's really nothing hard about it when you come to think about it.

How big the manapool will be doesn't matter. Percentages are always relative so they will even out obviously.

The idea in MoP is to try to further simplify healing a little bit. Instead of having INT being the best stat for everyone at all times (pretty much) they want to divide healing into throughput and longevity, thus making and INT-based stat only increase throughput while a SPI-based stat will be for longevity. this should overall be considered a good change in my opinion. That is because of the insane INT-scaling in cataclysm, you would hardly need any spirit in endgame for many of the healing speccs (or at least to a lesser extent) thus making healers go crazymode.

Also this change will in reality make it much worse to cast certain types of spells for certain types of specs. In explanation, an enhancement shammy will be able to cast 1/4 of a resto shammys heals before going oom.

Almost that, except as far as i understood, all priest specs will have 400,000 base mana, which is why they dont have the +400% skill; they have it naturally in their stat growth. This is because all priest specs would need it, so why not just have it inbuilt.

Yes, I think it pretty much will work that way.
Priests have 3 caster specs, so their base mana will be equal to their total mana. For some specs where the mana bar is supposed to be bigger in some specs and smaller in others, the base mana is low and the other specs get a 5x increase in their total mana pool through a talent.

As Holy, I'm glad they reduced mana costs (at least for the moment) since we still miss a mana regenerating spell/talent. Stacking Spirit like a mad was so boring.

mana costs are reduced, but so is mana pool (your mana pool will be always base, will not increase with gear, from what i understood).
from my pov nothing changes except that (since mana pool will not be increasing with gear while requirements for amount of healing will hopefully increase with new encounters) you will need to stack even more spirit

I like how someone compared mana pool to energy, that is in effect what they are doing. Agility currently gives rogues/ferals more attack power and some crit%, but it does not give them a larger energy bar. Currently Int gives spell power, spell crit, and a larger mana pool. They are changing it so Int will no longer give more mana, but at the same time they are reducing the spell costs. It would be nice if they let haste bump up mana regen for dps casters similar to how haste regens energy faster. Spirit will be more important for mana regen, while Int will still increase throughput of spells, it will not devalue regen as quickly as it currently does.

Priest base mana pool = 100,000 meaning that something that is 1% of your base mana is 1000 mana cost. The other classes have a 20,000 base mana pool meaning a 1% base mana cost is 200. So Blizzard needs to balance based off of that.

I'm not in beta so i can't confirm this, but i'm sure that Holy Light and Heal cost roughly the same right now.

Yep, wasn't thinking it terms of "base mana" all is right in the world again, thanks for helping my braindead moments, everybody!

The problem is that they are calling it "% of base mana" and not just "% of mana" or "% of total mana" - 'base' implies that your mana pool will become greater than the base somehow - but nothing increases your base mana in MoP - you have the same mana pool at all times. If they said "Greater heal costs 12/100 energy" that would make more sense to people - and that's effectively what they are doing to mana - turning it into an energy system.

Between turning mana into an energy system and giving everyone combo points (ie. Shadow orbs, holy power, maelstrom weapon, etc) we're all going to be rogues in MoP. I think that's pretty dumb, because if you talk to any top end rogues, the whole energy and combo system - wihle interesting in some mechanics (floating energy for burst, flow of combat through finishers, etc) for the most part they are both very clunky systems that could be improved. It's not some panacea to be applied en masse to every class to "fix" unplanned mana scaling and the like.

Caster hybrids that have a spec that does not or to a lower extent uses mana will get the passive ability to increase their manapool by 400% for specs that do use it.
Why those hybrids have such high base mana costs is because their base mana is still the 1/5 value of that increased manapool.
In order to compensate and make heals equally expensive they artificially scale the base mana cost to reflect the cost of pure mana users.
So 4% of 100k is still the same as 25% of 20k.
There's really nothing hard about it when you come to think about it.

How big the manapool will be doesn't matter. Percentages are always relative so they will even out obviously.

The problem with this is that, despite the percentage cost of heals, it's better to have lower mana costs on heals. The classes with the heals that cost the least amount of mana will benefit more from spirit.

This is a hypothetical:
-If I have a mana pool of 100, and my heal costs 5 mana (5%), and my spirit regens 2mp5, then i'll regen the cost of that heal in 12.5 seconds.
-If I have a mana pool of 1000, and my heal costs 50 mana (5%), and my spirit regens 2mp5, then i'll regen the cost of that heal in in over 2 minutes.

Correct me if I'm wrong, but wouldn't flat mana cost, as a value not a percentage, still be better when it's lower?

Sure it would be, but why even bother with such hypothetical scenarios?

Okay, well I'm not in beta right now... are all of the healer mana pools equal now? Because if they are, then yes, it wouldn't make sense to waste my time with that hypothetical, but the point would be valid if mana pools/costs aren't ~equal.

The problem is that they are calling it "% of base mana" and not just "% of mana" or "% of total mana" - 'base' implies that your mana pool will become greater than the base somehow - but nothing increases your base mana in MoP - you have the same mana pool at all times. If they said "Greater heal costs 12/100 energy" that would make more sense to people - and that's effectively what they are doing to mana - turning it into an energy system.

With "base mana cost" Blizzard could still use items which increase your mana pool (the 2% more mana meta gem for example) without increasing your mana costs